This invention relates to methods for the hydroxylation of aromatic substrates. In particular, this invention relates to a method for producing hydroxyaromatic compounds by the oxidation of aromatic substrates in the presence of oxygen, a catalyst, a proton source, and a non-gaseous reductant. The invention also relates to compositions for effecting said hydroxylation.
Phenol is among the most important industrial organic chemical intermediates, being used for the manufacture of thermoplastics and other resins, dyestuffs, explosives, agrochemicals, and pharmaceuticals. It is particularly important in the manufacture of phenol-formaldehyde resins used in the construction, appliance, and automotive industries, and in the manufacture of bisphenol A for epoxy and polycarbonate resins.
Despite its industrial importance, prior art methods for the production of phenol are non-selective, multi-step, and/or expensive. For example, benzene may be alkylated to obtain cumene, which in turn is oxidized to form cumene hydroperoxide. The hydroperoxide is cleaved using an acid catalyst to form phenol and acetone. Another industrial process using oxidation of toluene requires expensive starting materials. Older industrial processes such as the Raschig Hooker process require high energy input, and result in corrosive or difficult to dispose of wastes.
More recent processes for the production of phenols include the hydroxylation of aromatic substrates using hydrogen peroxide in the presence of a titanoaluminate molecular sieve, as disclosed in U.S. Pat. No. 5,233,097 to Nemeth et al., or in the presence of a hydrogen fluoride-carbon dioxide complex as disclosed in U.S. Pat. No. 3,453,332 to Vesely et al. U.S. Pat. No. 5,110,995 further discloses hydroxylation of phenol or phenol derivatives in the presence of nitrous oxide and zeolite catalyst. A multi-step process requiring partial hydrogenation of benzene, separation of the reaction products, oxidation of some of the reaction products, dehydrogenation, and other steps is disclosed in U.S. Pat. No. 5,180,871 to Matsunaga et al. U.S. Pat. No. 5,001,280 to Gubelmann et al., U.S. Pat. No. 5,110,995 to Kharitonov et al., and U.S. Pat. No. 5,756,861 to Panov et al. disclose oxidation of benzene to phenol by nitrous oxide in the presence of a zeolitic catalyst, with yields of up to about 16%.
While certain of these methods provide good yields, they still suffer from various drawbacks and disadvantages. In particular, nitrous oxide is expensive, and it is also a greenhouse gas that presents significant environmental concerns. Thus, despite the number of methods available to synthesize hydroxyaromatic compounds, there still remains a need for a process that is simple, high-yield, environmentally friendly, economical, and amenable to commercial scale-up.